What is “cling,” “skid” and “kick?”
“Cling” (AKA “skid” or “kick”) refers to a “bad hit” resulting from an excessive amount of throw, well beyond what is expected for a given shot. It is usually caused by a chalk mark appearing at the contact point between the CB and OB. See the following video for examples:
When the CB hits an OB with a cut angle or non-gearing spin, there is friction between the CB and OB at the point of contact that resists the relative motion between the balls. This is what causes throw (CIT or SIT), which is normal. A “bad hit” occurs when the amount of friction is greater than normal, usually because there is a chalk mark at the point of contact. In this case, the amount of throw (or ball hop and topspin loss in the case of a nearly straight follow shot) is larger than the typical amount. It is important to note that a “bad hit” has nothing to do with the shooter or the stroke … it is due solely to excessive friction occurring between the CB and OB at contact. People sometimes mistake a naturally large amount of throw as cling, especially if they are unaware of how throw varies with the type of shot (see throw effects and maximum throw). Again, cling is an amount of throw much greater than should be expected for a given shot and conditions. People also sometimes think that a “bad hit” results from the CB and OB actually clinging or sticking together for a longer time than normal. This is not the case, even though it might seem this way based on the reaction of the balls.
Cling can occur more often with old, beat up (e.g., from phenolic tip damage), scuffed (e.g., from miscues), and dirty balls, where portions of the ball surfaces might create more friction than other portions (especially when the suspect portions collect and hold chalk easily). However, cling also occurs with new, clean, and smooth balls. The primary cause for cling is a chalk mark or smudge (or a significant amount of chalk dust) appearing at the contact point between the CB and OB. Anytime you see chalk smudges on the CB, you should wipe them off (or ask for a referee to wipe them off if you are in the middle of a tournament game). Definitely wipe off the cue ball before each break shot or any time you have ball in hand. We have enough reasons to miss shots as it is without having to worry about excessive and unpredictable throw due to cling caused by chalk smudges.
Like throw, cling can be larger with slower shot speed. For more information, see throw speed effects.
Some people have suggested that cling can be caused directly by static electricity, but this is highly questionable (per logic, and per the test at the 3:44 point in NV D.16 – Pool ball cut-induced throw and cling/skid/kick experiment). Although, a possible explanation is that static (resulting from the balls sliding across the cloth, especially in dry conditions) could indirectly cause cling by somehow allowing chalk dust to collect on and stick to the balls more easily, especially if the cloth is very dirty with lots of chalk dust. This is also questionable, but it has been suggested in marketing materials of a snooker cloth manufacturer (per the study here). Throw could also be larger (for all shots) if the balls are “cleaned” or polished with a substance that alters the ball surface (e.g., by leaving a residue behind or by chemically etching or altering the surface), creating more friction. Some polishes/waxes or aggressive chemical cleaners (e.g., acetone) could have these effects. For more info, see ball cleaning and surface treatment. To see the effect (or lack of effect) of the brand of chalk on cling, see the chalk comparison resource page.
Some people have suggested that oils, from human hands, deposited on the balls as they are handled can help minimize the effects of cling. This could be the case, especially if the balls were previously “cleaned.” However, an excessive amount of oil could make it easier for chalk smudges to remain on the cue ball, which would result in more frequent cling. It has also been suggested that cling can occur more frequently on cloth that is new, thin, and slick because chalk smudges on the CB might tend to wear off less easily under these conditions (although, this is probably a very small effect). Cling might be more noticeable when playing with new and clean balls (e.g., in televised tournament conditions), where the amount of throw is less than with older and dirtier balls. Because the amount of throw can be less with ideal conditions, when cling does occur it can be strikingly noticeable.
George Onoda wrote an article illustrating how cling might be more likely with low-inside and high-outside english shots, where a new chalk mark might be more likely to end up at the ball contact point, but cling is probably more random than this suggests (due to previous chalk marks or smudges on the balls that happen to end up at the ball contact point, on any shot).
Throw, including cling, can be avoided by using a “gearing” amount of outside english. For more info, see: using outside english to limit or prevent throw and cling.
Cling is often talked about in relation to excessive throw of the OB with a cut shot, but it can also create a lot of trouble for slow-roll follow shots. The CB won’t follow the OB near as much as you would expect when there is cling. This video illustrates the effect:
For more information, see: “Throw Follow-up: Part IV: Follow Cling” (BD, October, 2014). Here are some other videos showing cling with follow and draw shots: HSV A.144 – Follow shot with chalk on the object ball to increase throw and spin transfer and HSV A.146 – Draw shot with chalk on the object ball to increase throw and spin transfer.
Also, object ball (OB) swerve has a slight effect on throw with follow shots. A follow shot will have slight OB swerve in the throw direction, effectively increasing the effective throw a tiny amount, but the effect is very small (see the end of TP A.24 for example numbers).
In the snooker world, the term “kick” is sometimes also used to refer to CB hop and its effect on OB motion. The effects of CB hop, along with video demonstrations, can be found on the ball hop resource page.
Here’s an example of purposely creating cling (with a chalk smudge) to help create a reverse bank angle:
The shot is demonstrated in Shot 731 here:
Here’s a fun proposition shot utilizing chalk-induced cling in a devious manner:
Other interesting shots utilizing cling can be found in Bob Jewett’s April ’09 BD article.
What can I do to help limit or prevent cling/skid/kick while playing?
You can wipe chalk marks off the CB every chance you get (for example, before each break, and with every ball in hand). That can eliminate many skids. You can also use more speed (for example going across the table instead of holding the ball) when possible, which will limit throw and skid. You can also use gearing outside english when possible, which will totally eliminate any possible throw or skid. You can also clean and polish balls with appropriate products because some cleaners and waxes create more throw than others.
If the CB and OB don’t stay in contact longer when “cling” occurs, what is it that happens instead?
“Cling” in the context of pool doesn’t actually mean the balls stick or “cling” together (although, it can seems like this based on the reaction of the balls when “cling” occurs). It just means there is more friction to resist sliding during contact.
The contact time between the balls, which is extremely small, depends only on how the balls compress (in the perpendicular or normal direction) during contact. The amount of sliding or friction between the balls during contact really doesn’t affect the contact time. With a cut angle, the CB tends to slide on the OB during contact. At smaller cut angles, the CB and OB slide at first but then “gear” together during contact. This happens sooner with more friction (e.g., with “cling”), but the contact time doesn’t change. At larger cut angles, the CB slides during the entire contact time, even with a more-than normal amount of friction.
What changes with increased friction is the amount of loss of relative sliding speed between the balls. With enough friction, the sliding disappears completely resulting in gearing motion. But again, this all happens during the normal ball compression and restitution.